1. Field of the Invention
The present invention relates to compositional analysis of a prepared sample. More particularly, the present invention relates to the preparation of samples such that they may be analyzed for their constituent components via laser induced breakdown spectroscopy (LIBS), or some similar technique of intensive analysis. Still more particularly, the samples may be prepared from any solid sample which is grindable, and, even more particularly, the samples may be prepared from the drilled cuttings of a subterranean wellbore.
2. Background and Related Art
There are many situations where it is necessary or desirable to obtain substantially instantaneous and/or immediate major and trace constituent analysis of a sample material. Sample materials may include geological samples, soil samples, powder metallurgy, ceramics, food, pharmaceuticals, and many other materials. There are many reasons why it would be necessary to test these materials for their composition of components. For example, when drilling a subterranean well for oil or gas, it may be necessary to periodically analyze the drilled cuttings that are returned to the surface. The data from the analyzed cuttings will describe the formation being drilled, and thereby allow the driller to adjust the drill plan accordingly.
Many analytical instruments are known in the art that can provide data, such as compound or elemental composition (absolute or relative), spectral response, and others. These various instruments all have in common their use of an energy source, which when used to perturb the sample, results in a measurable response. These instruments may be differentiated from each other by the type of energy source employed, i.e., beam, sound, spark, etc., and/or the manner of response detection. The specific output parameters resulting from the analysis, as well as the accuracy, repeatability, and timeliness of the results, are all highly dependent not only on the specific instrument type, but also on the specific form and physical characteristics, including integrity and homogeneity, of the sample being subjected to the analysis.
Often solid samples must conform to a specific shape to be analyzed by commercial analysis equipment. The process of conforming and binding the samples into a specific shape is generally known as palletizing or pressing, and is well known in the industry. Sometimes the typical pellet pressing equipment and binders are suitable for commercial analysis techniques, such as the scanning electron microscope (SEM) or X-Ray Fluorescence Spectroscopy (XRF) techniques. These techniques are non-intrusive (non-shock) or low impact, and do not rely on the physical integrity of the sample. In addition to the normal pellet pressing equipment for SEM/XRF samples, other preparation processes known in the art have also been employed for samples that experience no shock. Examples of prior art binders used in no-shock pellets include cellulose and lithium metaborate. However, the SEM/XRF-type sample preparation is inadequate where the sample must maintain physical integrity while experiencing external stress due to testing.
Recently, a process of applying a forceful laser beam to a target sample called laser induced breakdown spectroscopy (LIBS), or laser-induced plasma spectroscopy (LIPS), has been used as a tool for real-time, in situ, primary composition and impurity analysis. LIBS instruments are known as “intensive” devices because they apply energy in such a away as to produce mechanical stresses (via shockwaves) in the analyzed sample. The forceful beam or laser creates a force at impact that is induced not by collision, but by radiant energy. The radiant energy produces a mechanical form of kinetic energy.
LIBS is a useful method for determining the elemental composition of various solids, liquids, and gases. Referring now to FIG. 1, in the LIBS technique, a high power laser pulse 20 is focused on to a sample 30 to create a plasma or laser spark at test point or focal region 22. The spark in the focal region 22 generates a high density plasma plume 26 which produces and excites various atomic elements. Atomic emission 24 from the plasma may be collected with a collimating lens or fiber optics, and analyzed by a spectrograph and gated detector. The atomic spectral lines can be used to determine the elemental composition or the elemental concentrations in the sample. The analysis is similar to that performed by an inductively coupled plasma (ICP) analyzer, known to those skilled in the art.
LIBS can be applied using a variety of lasers, but typically excimers or pulsed Nd:Yag lasers are used. The high intensity laser pulse 20 interacting with sample 30 produces a plasma plume 26 that evolves with time from the point of impact 22 of the incident laser pulse. The laser pulse usually lasts for less than 20 nanoseconds (ns). Emissions 24 from plasma plume 26 are collected and analyzed by the detection system. Typically emission 24 is collected at some distance from sample 30 to reduce the effect on the data from self-absorption effects or surface effects. Ideally, the plasma created breaks down all the sample's chemical bonds and ionizes many of the constituent elements. The spectral emission occurs as a result of the subsequent relaxation of the constituent excited species.
For a more detailed explanation of LIBS devices and technology, reference is made to U.S. Pat. No. 5,751,416 to Singh et al., entitled Analytical Method using Laser Induced Breakdown Spectroscopy, which is incorporated herein by reference as though fully set forth in its entirety.
A major advantage of LIBS is that it is relatively easy to set up and is field deployable. LIBS can also be more accurate relative to SEM and other non-intrusive techniques. In practice, a very small amount of the sample material is ablated by the laser. However, the sample pellets undergo stresses during the LIBS process as the laser ablates the surface of the pellet. The force of the laser striking the surface of the pellet creates a shock, which destroys many samples produced via classical SEM preparation methods. For this reason, it has been difficult to explore new disciplines, such as geological and environmental analysis, with LIBS.
For geological and environmental analysis, soil and rock samples are the primary focus for processing. To be properly analyzed using LIBS, the soil and rock samples must, among other things, be homogeneous. Typically soil and rock samples are not naturally homogeneous, thus they must be processed into a homogeneous, properly-sized sample pellet. First, the soil and rock samples must be broken down into unconsolidated particles, usually by being ground to an extremely fine powder. A powdered sample may be defined as a sample of sufficiently ground particle size such that measurements taken by the analysis device will be representative of the whole sample. Such representative measurements require homogeneity of the sample to be retained throughout the pellet preparation process and ultimately preserved in the final pellet. The term powdered sample makes no reference to the concentration of liquids surrounding or in contact with the sample particles—the powder may be wet or dry. The powdered geological sample particles, on average, must be smaller than about 25 microns (10−6 m) in diameter, as is required by homogeneous sampling. Smaller particles are more desirable, but difficult to achieve via dry grinding methods and other methods used in conventional sample preparation methods.
Next, the powder is converted or reconstituted into a single solid unit. However, because the particles are so small, the sample is not easily bound such that it can withstand the stresses and shocks of the LIBS process. In addition to particle size, structural integrity depends largely on the binder used, and conventional prior art binders are not strong enough to withstand the laser shock from LIBS. Furthermore, it is difficult to mix conventional binders, such as cellulose and lithium metaborate, homogeneously with the powdered sample.
Due to LIBS detection limitations, it is important for measurement accuracy that the sample not be largely diluted by the binder such that the measurement is adversely destabilized. Conventional binders tend to limit the amount of material that is ablated because they remain present in the finished sample in significant quantities even after the pelletizing process. Therefore, a good binder will be present in low concentrations in the final sample pellet which allows for more uniform measurements.
Other characteristics of a good binder include: 1) the ability to cure the sample on demand, i.e., delay the commencement of curing, while at the same time having a short cure time without subjecting the sample to degradation from temperatures greater than 600° C. (1112° F.), 2) a minimal amount of epoxy required to bind the sample, and 3) consistent ablation characteristics of the sample during LIBS.
However, conventional binders and sample preparation techniques lack these characteristics, as well as other characteristics, including a prepared sample having a structural agent that maintains certain levels of structural integrity while being subjected to forceful beam analysis. The present invention overcomes certain deficiencies of the prior art.